Ultrasonic Cleaning Technology is a well-known method for cleaning different surfaces, i.e. glass and metal surfaces. Some vendors/suppliers of online Oil-in-Water (OiW) analyzer systems used this technology or method for cleaning optical sapphire windows in direct contact with produced water. Analyzers are normally installed after a degassing tank and a Compact Flotation Unit (CFU) for monitoring oil discharges to the sea. OiW monitor vendors are continually developing cleaning technologies that can operate and work efficiently at higher water pressures. This is especially important for process applications and for subsea monitoring.
In publication WO-2009/134145-A1, an inline optical probe is shown that can be used for measuring components in a fluid contained in a pipe or container. An acoustic transducer is acoustically coupled to the probe whereby the acoustic vibrations remove fouling from the optical window. The transducer emits an acoustic signal in the range of 20-30 kHz.
Publication WO-2011/128406-A1 discloses an imaging apparatus for the detection of oil droplets and other bodies in a flowing liquid. An ultrasonic transducer can be deployed for the cleaning of the optical window. The imaging system can be used in both inline and side-stream modes but the side-stream mode is only mentioned in passing without technical constructional details defining how the system could be implemented.
The operational pressure ranges for the use of these systems are not given in either of these documents.
New industrial requirements have specified the goal that such subsea analysis methods need to be able to be deployed at pressures exceeding 50 bar (5.0×106 Pa).
In general there are two different OiW monitor systems in operation, side-stream OiW systems that sample from a bypass line from the process line, and inline OiW systems where the measuring probe is placed directly in the process line. These and other similar systems have been tested by the inventor of the present invention at water-test-rigs.
For inline probe OiW monitor systems, the cleaning must be done at the inline process water pressure. For side-stream OiW monitor systems, it is possible to reduce the pressure by using two automatic control valves, so oil monitoring is done at process pressure and cleaning at low pressure, but the spill water released from the pressure reduction was collected in a pressure vessel under controlled operation. The conclusion from these tests was that existing ultrasonic cleaning systems can only operate effectively in lower water pressure applications (pressure below 20-25 bar, 2.0×106 Pa -2.5×106 Pa), and preferably at 10 bar (1.0×106 Pa) or below.
By definition, cavitation cleaning is most effective at lower pressure. Sound waves emitted from an ultrasound transducer are composed of an expansion mode and a compression mode. During the expansion mode the water molecules are pulled apart, and then are pressed together during the compression mode. If the expansion mode has sufficient energy to overcome the binding energy between the water molecules, a cavity, or bubble, is then produced. The compression mode then acts to implode the cavity which yields a gentle cleansing action to remove contaminants from surfaces. Most cleaning applications operate within the 20 kHz-250 kHz range, whereby a 25 kHz signal will produce 25,000 expansion/compression cycles per second. By example, a higher frequency will yield a smaller sized cavity and a more evenly distributed cavitation. Other factors influencing the cavitation efficiency include fluid density, viscosity, static fluid pressure and temperature. In general, if fluid density, viscosity and static fluid pressure are high, more energy is required to induce cavitation. Increasing temperature can be beneficial to point. Depending on the application, raising the temperature of the fluid or of a cleaning fluid to ca. 65-80% of its boiling point can assist in lowering the amount of energy to induce cavitation. Some ultrasound sensor systems may actually be designed to emit in the audible range, for example in the 12-20 kHz range, depending on the desired cleaning effect.
The development of such monitoring systems for subsea applications is very challenging. Although existing systems may be of adequate ability to fulfill the specifications set out for their use in lower pressure environments, of ca. 20-25 bar (2.0×106-2.5×106 Pa) or below, no solutions are given in the prior art that would enable one to solve the technical problem that is solved by the present invention.